When you load your page set a fullExtent variable:
private var fullExtent:Extent = new Extent(x,y,xx,yy,map.spatialReference);
Then create a function to use that when you want thje full extent shown.
Mine looks like this:
private function mapfullExtent():void
{
try{
if (fullExtent !=null)
{
map.extent = fullExtent;
}else{
navToolbar.zoomToFullExtent();
}
}catch(err:Error){
}
}